The invention relates to a sealing system for sealing a rotating shaft projecting through a sealing chamber.
A sealing system of this category has already been offered and sold by Sulzer Pumpen AG for some years under the sales name “J-Unit Cryogenic Sealing System”.
This sealing system is in particular used for sealing pump shafts of vertically arranged pumps for conveying for example liquefied natural gas (LNG). Liquefied natural gas has a temperature of approximately −160° C. so that standard sealing arrangements cannot easily be used. This sealing system is in particular characterized in that the cold liquefied natural gas is separated from the mechanical seal arrangement exerting the sealing function. This is achieved in that no liquefied natural gas is located in a sealing chamber adjoining the mechanical seal arrangement, but rather natural gas which has a much higher temperature than the liquefied natural gas. Standardized and thus tested and comparatively inexpensive shaft seals, in particular mechanical seals, can thus be used.
The named seal system is in this respect designed as a so-called add-on system. It is understood by this that the sealing system can be attached to a standard pump. For the named reasons, it is in particular suitable for use in the conveying of cold liquids, for example cryogenic fluids with a temperature below −80° C. It can, however, also be used at pumps which convey other liquids. A use is always advantageous when it has to be ensured that the conveyed liquid, which can, for example, be easily flammable or toxic, does not escape from the pump into the environment.
The named sealing system for sealing a rotating shaft projecting through a sealing chamber has a mechanical seal arrangement which is passed through by a barrier fluid at a barrier fluid pressure. A sealing fluid in gaseous state is present in the sealing chamber, in particular in gaseous form, whose exit should be prevented by the sealing system. The sealing fluid can be natural gas, for example.
The mechanical seal arrangement is supplied with barrier fluid by a barrier fluid pressure device. The barrier fluid pressure device has a cylinder in which a displaceable pressure piston is arranged which divides the cylinder into a region filled with sealing fluid in gaseous state and a region filled with and flowed through by barrier fluid. The pressure piston is thus arranged in the cylinder such that a sealing fluid space is formed between the pressure piston and a first cylinder end cover in which the sealing fluid pressure acts and a barrier fluid space is formed between the pressure piston and a second cylinder end cover which is flowed through by the barrier fluid. A force can be applied to the region filled with barrier fluid by means of the pressure piston and the barrier fluid pressure can thus be set. The barrier fluid pressure is in this respect greater by a fixed pressure difference than a sealing fluid pressure of the sealing fluid acting in the sealing chamber. The fixed pressure difference is achieved in that, on the one hand, as described above, the sealing fluid pressure and additionally an additional force act on the pressure piston. The additional force is applied by a spring which is arranged in the region of the cylinder filled with barrier fluid. The additional force in this respect acts on the pressure piston via a piston rod. The piston rod is likewise arranged completely within the region of the cylinder filled with barrier fluid.
A support piston and a spring retainer plate, between which the named spring is arranged, are arranged within the barrier fluid space. The spring retainer plate is for this purpose fixedly connected to the piston rod and can be axially displaced with respect to the cylinder. The support piston is axially fixed with respect to the cylinder and has a passage through which the piston rod projects so that the piston rod can be axially displaced with respect to the support piston. For the axial fixing of the support piston, the cylinder has a peripheral shoulder at which the support piston is supported against the force of the spring.